The 1971 Mariner Mars mission was planned to
consist of two spacecraft on complementary missions, but due to the
failure of Mariner 8 to launch properly, only one spacecraft was
available. Mariner 9 combined mission objectives of both Mariner 8
(mapping 70% of the Martian surface) and Mariner 9 (a study of temporal
changes in the Martian atmosphere and on the Martian surface).

Mariner 9 became the first spacecraft to orbit another planet. Launched
on May 30, 1971, the 506-kilogram (1,116-pound) spacecraft circled Mars
twice each day for a full year, photographing the surface and analyzing
the atmosphere with infrared and ultraviolet instruments. The spacecraft
gathered data on the atmospheric composition, density, pressure, and
temperature and also the surface composition, temperature, and
topography of Mars.

When Mariner 9 first arrived, Mars was almost totally obscured by dust
storms, which persisted for a month. But after the dust cleared, Mariner
9 proceeded to reveal a very different planet -- one that boasted
gigantic volcanoes and a grand canyon
stretching 4,800 kilometers (3,000 miles) across its surface. More
surprisingly, the relics of ancient riverbeds were carved in the
landscape of this seemingly dry and dusty planet. Mariner 9 exceeded
all primary photographic requirements by photo-mapping 100 percent of
the planet's surface and taking the first closeup photographs of the tiny
Martian moons, Deimos and
Phobos.